This invention relates to a screw mill and more particularly to a screw mill used for producing, for example, powder of calcium carbonate used as a bulking agent for paper.
Heretofore, several systems for grinding particles of small diameter have been known; which include, for example, the following:
(1) an impact and pressure-type system; this type of system is carried out, for example, by using a jaw crusher or an impact crusher;
(2) a pressure and shearing type system; this type of system is carried out, for example, by using a roller mill;
(3) an autogenous grinding type system, (for example, striking, pressing, rolling, and high speed rolling and impacting type system); this type of system is carried out by using an aerofall mill, a jet mill, a rolling cylinder mill, an attrition mill or a super-micron mill;
(4) a type of system which uses a grinding medium; this type of system is carried out by using a ball mill, a rod mill, a vibration ball mill or an attrition mill;
(5) a combination of the system types (1)-(4).
However, none of these types of systems can easily control the grain size nor can they manufacture powder grains each having spheroidicity of less than 0.3 which is necessary to be suitable for use as a bulking agent for paper.